1. Technical Field
The present disclosure relates to hollow core photonic crystal fibers (HC-PCFs) and more specifically to a system and method of improving fiber coupling and projection of HC-PCF modes onto a detector to suppress unwanted multiple-mode beating noise.
2. Introduction
The concepts disclosed herein relate to the development of a wavelength-stabilized laser transmitter for the Active Sensing of CO2 Emissions over NASA's Nights, Days, and Seasons (ASCENDS) mission. To ensure a 1 parts per million by volume (ppmv) CO2 measurement, the laser wavelength must be stabilized to sub-MHz accuracy because of the high slopes on the sides of the CO2 absorption line near 1572.33 nm to be measured. The locking accuracy exceeds the current state of the art being developed for the telecom industry (±1 GHz) by over 2000 times.
The 1572.33 nm CO2 line selected for measurement turns out to be the locking reference of choice because there are no better references available near this line. The inventors have, for the first time, demonstrated <0.21 MHz drift of a “reference” distributed feedback laser diode (DFB-LD) locked to this absorption line of CO2 in a gas cell using a frequency modulation (FM) technique based on external phase modulation and phase-sensitive detection. Due to the low CO2 absorption strength and low cell pressure (˜40 mbar) needed to narrow the linewidth, a long cell path length (18 m for the cell) had to be used to gain high slope of the error signal. It is important to minimize the predominant noise for the locking system. The noise is the time-varying residual amplitude modulation (RAM) stemmed from multi-path interference (MPI) along the optical path, particularly in the gas cell. When the gas cell output beam contains multiple spatial modes, the time-varying multi-mode beating often leads to unwanted RAM in the detector signal. It is highly desirable to use gas filled hollow-core photonic crystal fibers (HC-PCFs) to make the cells compact, light weighted, reliable, and conveniently fiber coupled and sealed.
Various techniques have been used to build all-fiber HC-PCF gas cells. Both ends of a gas filled HC-PCF can be spliced to conventional single mode fibers (SMFs) with a filament splicer, an arc splicer or even a CO2 laser. However, the SMF to HC-PFC splice suffers ˜1 dB excessive insertion losses that are linked to the formation of a recess in the end face of the HC-PCF when heated in the splicer. The losses resulted from the action of surface tension along the many glass-air interfaces within the holey structure. Furthermore, such a splicing suffers a 4% Fresnel reflection at the normal-cleaved air-silica splice interface, causing undesirable MPI in the cell.